only one brake light lights up on the left side.

Check for corrosion in the socket and check to see if you have power to the bulb and a good ground.

The chassis ground is not connected properly (black cable to chassis) as well as the cable to the engine too-- the light fixture grounding tabs to make a connection to the ground and the screws make an electrical connection...your battery is old and would not survive the dead short for three seconds, then start the car test, get a new one...they will degrade ten percent per year and a a ten year old needs to be cashed in for a new one~

Its one light bulb judge the battery is not the issue

If the main ground was no good from the battery to the chassis nothing would work. Like the vehicle would have no electrical power then all your light bulbs would be out not just one.

solid ground and solid power is the issue, I'm thinking globally, you're thinking locally...both are correct.~

I still disagree your always preaching about ground issues when it comes to a vehicle not starting. dont you think if the main grounds were bad/loose/whatever the vehicle wouldnt start and you would have many other large issues. when it comes to one system and in this case one tiny part in one system it is not a battery issue. You not gonna change my mind on this one.

kbro, you're at liberty to brag about the teachings of Nicolai Tesla or Einstein or whichever makes you feel grounded- Quite. your mind was made up already when you tasted your first piece of candy!

swap bulb holders left to right and see if problem goes with holder. in particular, all wires for whole tail light enter the turn signal holder to be distributed via contacts on it. swap it and see if problem goes with holder. while holders are removed pay attention to burned spots in the tail light where contacts touch and look at holders for bent contacts. see if that gets you anywhere. if not break out the test light with lights on and see if you're even getting power back there. good luck

I had the same problem some time its on and some times not check the brake pedal switch. might not be fully engaging if worn i put a small plug on end of switch to give more reach

If switch makes contacted with firewall, all the light do weird stuff